Hey guys need some help. I was an avid flier from the mid 80's thru mid 90's, but family problems and other things caused me to put my flying on hold till a month or so ago.
I ran YS-45's on all my planes and had them put away till now. I bought a sport trainer to get the thumbs working again but continue to have problems with my old YS I installed on the plane. I was expecting some engine problems but this is making me crazy.
I removed the carb and pump assemblies and soaked them in denatured alcohol to clean everything up. Installed new O-rings on the needle body and needle, new diaphram, plunger and spring for the pump, also replaced check valve and thoroughly checked all componets over. The engine fired right up and runs great, the problems are on the low end adjustment. To make the engine transition from low to high end, I have to run the pump adjustment screw all the way clockwise into to the pump body, other wise the engine runs extremely rich and blubbers and loads up during the transition to high speed. The tank and fuel lines are holding pressure and the engine is strong on the top end. When it is time to refuel after a flight the engine is very difficult to get restarted. Many times I will have to back out the pump adj. screw a bit in order to get it started, It acts as if the pump is not allowing fuel to the carb at these times.
I ran these engines for years and never encountered this type of problem. Actually the ys was probably the most bullet proof engines I ever owned. Get them set initally and never had to be tweaking them as with other engines.
I also took a pump and carb from one of my other ys's and rebuilt it, and am still having the same problems, acting identical to the first carb and pump.
I really didnt want to send the engine away for repair, I just feel I am missing something on the trouble shooting on my part. Any thoughts or advice for repair would be greatly appreciated

It seems strange to me that the regulator would need closed to run when cold and then opened to restart. It's almost like there is debris being flushed through it or the plunger might be a little sticky when cold. When you replaced the plunger did you clean out the bore it rides in?

Petec, thanks for a response, and yes I cleaned everything thoroughly. As an update to the problem, today I pulled a ys from another plane that I am not flying. Installed the rebuilt pump and carb from the other engine to the engine I just pulled off. This engine fired right up, low end was a bit rich, adjusted the pump screw inward one half turn and low speed idle was set. Engine sits and idles for minutes at a time, then transition up to high speed, smooth as silk, just as it should.
The one thing I did not change on the first engine was the rear crankcase cover gasket, I am now thinking there was a leak in that particular gasket somehow causing the over-rich low end situation. I plan on bench running the first engine now that I have changed the gasket and see what happens. Hard for me to believe that was the problem, but have seen stranger things. The pump and carb function flawless on the second engine.
I will make a post later on here if the gasket took care of the problem on the first engine....Howard

I don't think a leaky rear crankcase cover gasket would cause an engine to run rich, would it? Wouldn't that make it run lean?

I'd take a look at the ring in the engine that won't run right. Sometimes when a motor hasn't been run in a while the ring can stick and cause problems.

Also, if the engine stopped the last time it ran with a valve spring compressed, and it has sat for that long time without being rotated, the spring may have lost a lot of it's tension, and that could cause it to miss and run rough. Just a thought.

David

Howard,

How do the bearing feel in the problem engine? You said the fuel system holds pressure so I would not think the rear gasket would be an issue, but then again as you stated so well, I have seen stranger things.

With the 2 stroke YS engines there are so few things to go wrong that the bearings mey just need a bit of lubrication to come back to life or replaced.

I know this is an older post but it’s something that’s always coming up.
I see a couple of guys struggling for some time trying to get an engine running so that it would be air worthy. Finally my curiosity has me introduce myself and I find out its been sitting a few years but it ran great when it was put away, than ask all the stupid questions like “how old is the fuel?” I than ask “is that a new glow plug?” They usually say “no, but it starts and runs.” I say humor me and try a fresh one.
Problem solved.

I have even fallen in the same trap myself. I have a YS 45 RE that I fly regularly, one day it just won’t idle. I start tweaking with the pump screw and it just will not settle down.
Then I remembered, TRY ANOTHER PLUG dummy! That was it. Now I get to find the old settings.

Plugs can glow, look uniform and intact, (sometimes looks gray) but in fact they are finished.

The moral of the story is, if you have an engine that was running fine the last time it ran and now suddenly it won’t, try the simplest tune up first, a new glow plug.